Global Positioning System (GPS) and the associated receivers are widely used in agricultural applications. One such application involves the leveling of agricultural land to reduce water accumulation or run-off caused by bumps and troughs in the land.
In order to map the undulations in the land a farm vehicle, e.g., a tractor or truck is typically driven back and forth across the field 100 as depicted by the line 102 in FIG. 1 to create a map in the xyz dimensions using a GPS receiver. For improved accuracy it is common to make use of the relative timing of signals transmitted from a number of satellites visible to the receiver and process the results using a technique referred to as Real Time Kinematics (RTK) to obtain highly accurate position fixes. For a better understanding of the concepts of GPS and RTK, a brief discussion is included below. One prior art technique involves using the resultant three dimensional map to produce a best-fit plain 200 as illustrated in FIG. 2.
In another prior art mapping approach, instead of using a satellite receiver to monitor the position of the vehicle, a laser plain is first created using laser beams, whereafter a vehicle is driven back and forth with a laser receiver to see if the height of the receiver and thus the height of the ground varies relative to the laser plain. This is discussed in U.S. Pat. No. 4,807,131 to Phil Clegg entitled Grading System. Once the mapping is complete, scrapers are pulled over the land to selectively scrape away the high sections and dump soil into the low sections to achieve a less undulating surface.
FIG. 3 shows a cross section through a typical prior art box scraper 300, which defines a bucket or housing 302 for receiving the soil, also referred to herein as dirt. In order not to limit the application to a particular type of scraper the term scraper and earth scraper will also be used herein. The scraping function is performed by a blade 304, controlled by pistons (not shown), which lower and raise the blade through an opening in the bottom of the housing 302. As the scraper 300 is pulled along with the blade 304 lowered, soil is scraped off the surface of the ground and passes over the top of the blade into the housing 302 where it accumulates. In order to dump this soil onto the ground at the desired locations a pusher 310 controlled by pistons 312 is activated to push the soil out of the front 314 of the housing 302 under the blade. Since the depth to which the blade can scrape is limited and also depends on the hardness of the ground, several passes may be necessary to remove and dump enough soil to level the piece of agricultural land or road that is being leveled. For instance, it is quite possible that one may have to scrape 5 feet deep, each scraping run removing only about 6 inches of soil from the surface and consequently requiring some 10 runs to remove the 5 feet of dirt. Typically a control algorithm is used to position the blade that governs when and how deep to scrape and when and how much soil to dump. Thus, it will be appreciated that the cost involved in leveling the bumps and troughs in the land is related to the amount of dirt that has to be moved since it corresponds to the time, fuel, and wear and tear on the machinery. Therefore any improvement in efficiency in performing the transfer of dirt from one location to another translates into a significant cost saving. While the above description refers to the leveling of agricultural land the process and system discussed in this application is not limited to agricultural land and could be used, for example, to level the ground for new road surfaces. Also the term leveling is used here to refer to the evening out of troughs and bumps to provide a smoother surface irrespective of what the angle of the resultant plain is. Thus it is not limited to flat plains but will quite commonly involve the forming of a smooth, less undulating slope.
In order to reduce the number of passes that has to be made by a scraper, one approach has been to connect two or more scrapers behind one another (also referred to as connection in series or in tandem) and switching from one scraper to the next when the first one is full. To achieve this the blade's height in each scraper is moved to control the depth to which it scrapes or the rate at which soil is dumped from the scraper.
Due to cost considerations, this solution is not without its issues, however as satellite receivers used for this purpose need to be highly accurate and are usually expensive.
The present invention seeks to address some of these issues by providing a new system and method of monitoring towed vehicle positions.
As mentioned above, some of the concepts of GPS, DGPS and RTK are laid out here for a better appreciation of the techniques and issues involved in obtaining positional information using a system of satellites.
In the available art, the Global Positioning System (GPS) is a system of satellite signal transmitters that transmits information from which an observer's present location and/or the time of observation can be determined. Another satellite-based navigation system is called the Global Orbiting Navigational System (GLONASS), which can operate as an alternative or supplemental system.
The GPS was developed by the United States Department of Defense (DOD) under its NAVSTAR satellite program. A fully operational GPS includes more than 21 Earth orbiting satellites approximately uniformly dispersed around six circular orbits with four satellites each, the orbits being inclined at an angle of 55.degree. relative to the equator and being separated from each other by multiples of 60.degree. longitude. The orbits have radii of 26,560 kilometers and are approximately circular. The orbits are non-geosynchronous, with 0.5 sidereal day (11.967 hours) orbital time intervals, so that the satellites move with time relative to the Earth below. Generally, four or more GPS satellites will be visible from most points on the Earth's surface, and can be used to determine an observer's position anywhere on the Earth's surface, 24 hours per day. Each satellite carries a cesium or rubidium atomic clock to provide timing information for the signals transmitted by the satellites. An internal clock correction is provided for each satellite clock.
Each GPS satellite continuously transmits two spread spectrum, L-band carrier signals: an L1 signal having a frequency f1=1575.42 MHZ (nineteen centimeter carrier wavelength) and an L2 signal having a frequency f2=1227.6 MHZ (twenty-four centimeter carrier wavelength). These two frequencies are integral multiplies f1=1,540 f0 and f2=1,200 f0 of a base frequency f0=1.023 MHZ. The deployment of additional frequencies is being planned by the DOD.
The L1 signal from each satellite is binary phase shift key (BPSK) modulated by two pseudo-random noise (PRN) codes in phase quadrature, designated as the C/A-code and P-code. The L2 signal from each satellite is BPSK modulated by only the P-code. The nature of these PRN codes is described below.
Use of PRN codes allows use of a plurality of GPS satellite signals for determining an observer's position and for providing the navigation information.
A signal transmitted by a particular GPS satellite is selected by generating and matching, or correlating, the PRN code for that particular satellite. Some of the PRN codes are known and are generated or stored in GPS satellite signal receivers operated by users.
A first known PRN code for each GPS satellite, sometimes referred to as a precision code or P-code, is a relatively long, fine-grained code having an associated clock or chip rate of f0=10.23 MHZ. A second known PRN code for each GPS satellite, sometimes referred to as a clear/acquisition code or C/A-code, is intended to facilitate rapid satellite signal acquisition and hand-over to the P-code and is a relatively short, coarser-grained code having a clock or chip rate of f0=1.023 MHZ. The C/A-code for any GPS satellite has a length of 1023 chips or time increments before this code repeats. The full P-code has a length of 259 days, with each satellite transmitting a unique portion of the full P-code. The portion of P-code used for a given GPS satellite has a length of precisely one week (7.000 days) before this code portion repeats.
Accepted methods for generating the C/A-code and P-code are set forth in the document ICD-GPS-200: GPS Interface Control Document, ARINC Research, 1997, GPS Joint Program Office, which is incorporated by reference herein.
The GPS satellite bit stream includes navigational information on the ephemeris of the transmitting GPS satellite (which includes orbital information about the transmitting satellite within next several hours of transmission) and an almanac for all GPS satellites (which includes a less detailed orbital information about all other satellites). The transmitted satellite information also includes parameters providing corrections for ionospheric signal propagation delays (suitable for single frequency receivers) and for an offset time between satellite clock time and true GPS time. The navigational information is transmitted at a rate of 50 Baud.
A second satellite-based navigation system is the Global Orbiting Navigation Satellite System (GLONASS), placed in orbit by the former Soviet Union and now maintained by the Russian Republic. GLONASS uses 24 satellites, distributed approximately uniformly in three orbital planes of eight satellites each. Each orbital plane has a nominal inclination of 64.8.degree. relative to the equator, and the three orbital planes are separated from each other by multiples of 120.degree. longitude. The GLONASS satellites have circular orbits with a radii of about 25,510 kilometers and a satellite period of revolution of 8/17 of a sidereal day (11.26 hours). A GLONASS satellite and a GPS satellite will thus complete 17 and 16 revolutions, respectively, around the Earth every 8 days. Instead of making use of one set of carrier frequencies for all of its satellites and transmitting information on the carrier waves to identify the various satellites, as in the GPS system, the GLONASS system uses two carrier signals L1 and L2 with frequencies of f1=(1.602+9 k/16) GHz and f2=(1.246+7 k/16) GHz, where k(=1, 2, . . . 24) is the channel or satellite number. These frequencies lie in two bands at 1.597-1.617 GHz (L1) and 1,240-1,260 GHz (L2). The L1 code is modulated by a C/A-code (chip rate=0.511 MHZ) and by a P-code (chip rate=5.11 MHZ). The L2 code is presently modulated only by the P-code. The GLONASS satellites also transmit navigational data at a rate of 50 Baud. Because the channel frequencies are distinguishable from each other, the P-code is the same, and the C/A-code is the same, for each satellite. The methods for receiving and demodulating the GLONASS signals are similar to the methods used for the GPS signals.
The present application will also refer to a Satellite Based Augmentation System or SBAS, which will refer to a GPS or GLONASS, and to any other compatible satellite-based system that provides information by which an observer's position and the time of observation can be determined, all of which meet the requirements of the present invention.
From the above discussion it will be clear that a Satellite Based Augmentation System (SBAS) such as the Global Positioning System (GPS) or the Global Orbiting Navigation Satellite System (GLONASS), uses transmission of coded radio signals, with the structure described above, from a plurality of Earth-orbiting satellites. A SBAS antenna receives SBAS signals from a plurality (preferably four or more) of SBAS satellites and passes these signals to an SBAS signal receiver/processor, which (1) identifies the SBAS satellite source for each SBAS signal, (2) determines the time at which each identified SBAS signal arrives at the antenna, and (3) determines the present location of the SBAS satellites.
The range (r.sub.i) between the location of the i-th SBAS satellite and the SBAS receiver is equal to the speed of light c times (.DELTA.t.sub.i), wherein (.DELTA.t.sub.i) is the time difference between the SBAS receiver's clock and the time indicated by the satellite when it transmitted the relevant phase. However, the SBAS receiver has an inexpensive quartz clock which is not synchronized with respect to the much more stable and precise atomic clocks carried on board the satellites. Consequently, the SBAS receiver estimates a pseudo-range (pr.sub.i) (not a true range) to each satellite.
After the SBAS receiver determines the coordinates of the i-th SBAS satellite by demodulating the transmitted ephemeris parameters, the SBAS receiver can obtain the solution of the set of the simultaneous equations for its unknown coordinates (x.sub.0, y.sub.0, z.sub.0) and for unknown time bias error (cb). The SBAS receiver can also determine velocity of a moving platform.
The discussion below refers specifically to GPS but is applicable to any satellite navigational system.
Differential Global Positioning System (DGPS) is a technique that significantly improves both the accuracy and the integrity of the Global Positioning System (GPS). The most common version of DGPS requires high-quality GPS “reference receivers” at known, surveyed locations. The reference station estimates the slowly varying error components of each satellite range measurement and forms a correction for each GPS satellite in view. This correction is broadcast to all DGPS users on a convenient communication link. Typical ranges for a local area differential GPS (LADGPS) station are up to 150 km. Within this operating range, the differential correction greatly improves accuracy for all users, regardless of whether selective availability (SA) is activated or is not. This improvement in the accuracy of the Global Positioning System (GPS) is possible because the largest GPS errors vary slowly with time and are strongly correlated over distance. DGPS also significantly improves the “integrity” of GPS for all classes of users, because it reduces the probability that a GPS user would suffer from an unacceptable position error attributable to an undetected system fault. Expected accuracies with DGPS are within the range from 1 to 5 meters.
Most DGPS systems use a single reference station to develop a scalar correction to the code-phase measurement. If the correction is delivered within 10 seconds, and the user is within 1000 km, the user accuracy should be between 1 and 10 meters.
Instead, a network of reference stations can be used to form a vector correction for each satellite. This vector consists of individual corrections for the satellite clock, three components of satellite positioning error (or ephemeris), and parameters of an ionospheric delay model. The validity of this correction still decreases with increased latency or age of the correction. However, compared to a scalar correction, a vector correction is valid over much greater geographical areas. This concept is called wide area DGPS, or WADGPS. Such network can be used for continental or even world-hemisphere coverage, because it requires many fewer reference stations than a collection of independent systems with one reference station each, and because it requires less communication capacity than the equivalent network of LADGPS systems.
Users with very stringent accuracy requirements may be able to use a technique called carrier-phase DGPS or CDPGS. For CDGPS, the definition of long baseline is arbitrary, but usually refers to baseline lengths exceeding 20 km and up to 100 km. Lines in excess of 100 km may be referred to as very long baselines.
These users measure the phase of the GPS carrier relative to the carrier phase at a reference site; thus achieving range measurement precision that is a few percent of the carrier wavelength, typically about one centimeter. These GPS phase comparisons are used for vehicle attitude determination and also in survey applications, where the antennas are separated by tens of kilometers. If the antennas are fixed, then the survey is called static, and millimeter accuracies are possible, because long averaging times can be used to combat random noise. If the antennas are moving, then the survey is kinematic, and shorter time constants should be used with some degradation of accuracy. The latter is also referred to as Real Time Kinematic or RTK.
The above discussion can be found in “Global Positioning System: Theory and Applications”, Volume II, Chapter 1, by Bradford W. Parkinson and James J. Spilker Jr., published by the American Institute of Aeronautics and Astronautics, Inc. in 1996.
As mentioned above, the present invention deals specifically with the monitoring and adjusting of a scraper blade on a box scraper or other earth scraping implement.